The Gould Property Co. is planning to break ground next month on a not-so-speculative spec office building by Mount Vernon Square despite a prolonged wave of uncertainty surrounding D.C.'s office market.

Gould is hoping, in moving forward with the 11-story office building at 600 Massachusetts Ave. NW, to tap into what it sees as a strong tenant demand to move up to newer, more efficient D.C. office buildings, even as other developers who took the same gamble are still awaiting their first tenants. Gould representative Garrett Preis declined to discuss leasing prospects for the 400,000-square-foot office building but said he is confident tenants will have arrived by the time construction wraps up roughly two years from now.

"We decided to move forward with a great project in a great location," Preis said. "Obviously we're hoping to attract quality tenants that appreciate a well-thought-through building that hopefully meets their needs."

Gould is, in part, playing into a tale of D.C.'s two cities, in a manner of speaking. The District's vacancy rate now hovers at 11.8 percent, according to JLL, with more than 13.6 million square feet of vacant space on the market. That's up from 11 percent a year ago, inflated in no small measure by companies and federal agencies alike shedding space in the name of greater efficiency. Still, that downsizing has benefited developers willing to move ahead with new buildings, which are often the preferred replacement for tenants looking for more efficient, updated space.

Gould's not the only developer willing to take a chance on spec development these days, with the Republic Properties Corp. and The Shooshan Cos. also willing to take the risk. At the same time, Trammell Crow Co. and StonebridgeCarras are still awaiting their first tenants in NoMa, as is Monday Properties at 1812 N. Moore St. in Rosslyn.

What's more, Preis is hedging a bit by moving ahead at 600 Mass, for which it was recently awarded a permit to build, instead of going with a larger, 620,000-square-foot tower at 900 New York Ave. NW by CityCenterDC.

"Six hundred Massachusetts is a little smaller project, and there's a lot of excitement around it," Preis said.